Law & Order - Dick Wolf was able to make a franchise massively huge that I don't know if any could compare. Law & Order begot SVU, a show that completely stands on it's own. Law & Order Criminal Intent has had to use various shticks to sustain itself like rotating leading characters and staff overhauls in recent years. The original having new faces constantly makes sense when the series lasts 19 years...we miss you Lenny! Wolf's had his failure as well with Trial By Jury, but I'm sure he loses no sleep over it with his other babies all grown up and successful. I've seen L&O: UK on imdb and wonder how far he'll be able to stretch this before something breaks.
Then you have CSI. The original may not survive the loss of three primary characters. I feel sorry for the actress playing Riley...her character was completely lost in the shuffle when everyone else started dropping like flies. CSI: Miami is truly a joke to me...no science to drag down the average viewer just crappy dialog, unrealistic situations (not just Calley running in 6" stilettos, but every couple weeks something huge blows up), and somehow it's the cream of the crop from Mr. Bruckheimer. CSI: NY took me a while to warm up to after being let down by Miami. These aren't going away anytime soon.
Now I see NCIS is bringing on NCIS: LA - Congratulations you officially have a ridiculous amount of letters in your acronym.
Spin offs are almost as mind boggling, but at least they have the courage to call the new show by another name. They still stretch the franchise, but it seems closer to riding coat tails rather than the rub and tug the ones above seem to be. Angel spinning off of Buffy seemed to be successful. Joey...not so much. The jury is still out on Grey's Anatomy's creation of Private Practice. How Cleveland coming from Family Guy will fair is really going to be a test of Seth McFarland's cult like following.